Solid floor insulation above or below slab

[tamaki383]

I am installing floor insulation for a ground floor (bungalow) bathroom, that will have wet underfloor heating. I have already cut up and removed the old concrete slab and dug out soil/rubble until the top of the footings were exposed.

I am now faced with a decision of whether to have the insulation underneath the new concrete slab, or lay the new slab down first. In either case the DPM would go below the insulation / slab and the screed with UFH pipes would go at the very top, and there will be a perimeter insulation strip just for the bathroom.

The right answer isn't obvious to me because this is an unusual situation. The bathroom is a small one, and the rest of the solid floor in the house is not (and will never be) insulated to the same standard. The bathroom floor insulation that I am installing is mainly to ensure that the UFH is reasonably efficient. Since its a bathroom, and the rest of the house has mediocre insulation there isn't a good reason to have the small area of UFH in the bathroom on all the time, so could having thick slab to heat up be undesirable? Since during the relatively long period of time is it heating up, heat will be wasted through the walls and ceiling? In which case am I better off putting the insulation on top of the slab?

 

[tamaki383]

Also if I decide to have the insulation above the concrete, will the thin screed be able to safely support point loads like bath and wall hung toilet feet? The insulation is EPS which should be OK beneath a thick slab to spread the load but I'm not sure if the screed would be strong enough to avoid 'punching through'.

 

[Diver Fred]

The insulation needs to be underneath the slab. The UFH will heat the slab and that will then become a 'Thermal Store' and behave similar to the way a night storage heater. It will take a time to warm up but also take time to cool down.
If you can insulate the sides of the slab against the external walls, improves the efficiency.
Make sure you have completed any and all penetrations through the floor before pouring the slab. Insulate around such.

 

[Notch7]

I am installing floor insulation for a ground floor (bungalow) bathroom, that will have wet underfloor heating. I have already cut up and removed the old concrete slab and dug out soil/rubble until the top of the footings were exposed.

I am now faced with a decision of whether to have the insulation underneath the new concrete slab, or lay the new slab down first. In either case the DPM would go below the insulation / slab and the screed with UFH pipes would go at the very top, and there will be a perimeter insulation strip just for the bathroom.

The right answer isn't obvious to me because this is an unusual situation. The bathroom is a small one, and the rest of the solid floor in the house is not (and will never be) insulated to the same standard. The bathroom floor insulation that I am installing is mainly to ensure that the UFH is reasonably efficient. Since its a bathroom, and the rest of the house has mediocre insulation there isn't a good reason to have the small area of UFH in the bathroom on all the time, so could the thermal mass of the slab-above-insulation be undesirable? Since during the relatively long period of time is it heating up, heat will be wasted through the walls and ceiling? In which case am I better off putting the insulation on top of the slab?

electric or wet?

personally I think that a decent 65mm to 75m of unbonded screed is fine as a thermal heat store

for electric underfloor heating, Ive heard that doing it as a mat under the tiles is good as the heat comes through quick but also heat travels down into the screed

for wet it works well to clip it to the insulation and screed over

I agree with Diver Fred on using the whole of the concrete for a thermal store -but that rather depends on your usage, such a large mass would take a lot of time to alter the room temperature so you would need to have it on a lot to be confident its going to be warm -heating concrete +screed would take hours

 

[tamaki383]

The insulation needs to be underneath the slab. The UFH will heat the slab and that will then become a 'Thermal Store' and behave similar to the way a night storage heater. It will take a time to warm up but also take time to cool down.

This is the crux of the issue. I think I understand the concept of thermal store, and that it would make a lot of sense with underfloor heating in a large area of a modern well insulated house, that you would have active at a very low temperature for most of the time. But in my case I will have UFD in a small area of an otherwise badly insulated bungalow (1970s standard). The UFD here is more of a barefoot comfort/luxury feature and to help dry out the bathroom, to be activated as and when needed. If I leave it running most of the time it will probably waste energy through the fabric of the house which is otherwise poorly insulated. To optimise energy efficiency in houses that are not insulated to modern standards and can't easily be upgraded to those standards you are better off with room controls, low thermal mass and rapid response times. That's my understanding of the issue, have I got it right?

 

[Diver Fred]

In the end with 'Wet' UFH the tiles will have to be on a solid surface - i.e. a 'slab' that will, at a minimum, I'd say 50mm thick. With you having already removed the existing concrete floor I wonder how thick you are planning the replacement slab to be.
You should be planning to have a compacted base of stone, compacted soft sand 40-50mm thick,DPM, insulation that should be as thick as possible - minimum 50mm, ideally 100mm or thicker and then the UFH 'slab' if that you are planning to be more than 50mm then you should be doing 2 pours - first one to bring the slab up to 50/60mm below final floor level - UFH piping, and final floor pour.

A final word of advice if or when you do the wet UFH space the pipes at 100mm centres rather than 150mm to future proof the floor.

 

[tamaki383]

In the end with 'Wet' UFH the tiles will have to be on a solid surface - i.e. a 'slab' that will, at a minimum, I'd say 50mm thick.

Either way, this would be the screed on top of either the concrete slab or insulation.

With you having already removed the existing concrete floor I wonder how thick you are planning the replacement slab to be.

Approx 4 inches for the concrete.

You should be planning to have a compacted base of stone, compacted soft sand 40-50mm thick,DPM,

Got this done today. I now have a pit lined with DPM, ready to receive either the concrete pour or slabs of insulation.

insulation that should be as thick as possible - minimum 50mm, ideally 100mm or thicker

I have room for 150mm of insulation, planning to use high performance EPS with a conductivity of around 0.03.

UFH 'slab' if that you are planning to be more than 50mm then you should be doing 2 pours - first one to bring the slab up to 50/60mm below final floor level - UFH piping, and final floor pour.

Well yes basically. Whichever I go there will be a 4 inch concrete pour, and at least one screed pour. Although the screed is complicated because I need to incorporate drainage slopes (bathroom will be a wet room). Just need to decide if the insulation is going underneath the concrete or if it will be sandwiched between the concrete and the screed.

A final word of advice if or when you do the wet UFH space the pipes at 100mm centres rather than 150mm to future proof the floor.

Good point, although this task is some way down the line. I would be consulting the UFH vendor in any case on details like this.